The Internet has dramatically increased the potential for data, voice, and video services for customers. Existing circuit-switched telephony systems, however, do not provide the foundation to support the growing need for bandwidth and new services required by both residential and business consumers. As a result, integrated access devices have been introduced to support Internet and related technologies as well as standard telephony service.
Integrated access devices often employ asynchronous transfer mode (ATM) functionality to multiplex data, voice and video traffic together onto a single network. ATM is a connection-oriented packet-switching technology in which information is organized into small, fixed length cells. ATM carries data asynchronously, automatically assigning data cells to available time slots on demand to provide maximum throughput. Compared with other network technologies, ATM provides large increases in maximum supported bandwidth, support for multiple types of traffic such as data, video, and voice transmissions on shared communication lines, and virtual networking capabilities, which increase bandwidth utilization and ease network administration.
Integrated access devices are typically implemented in a card shelf configuration with functionality of the device distributed between discrete line and switch cards connected over a backplane. The line cards provide the service interfaces for the different traffic types and segment and reassemble (SAR) time division multiplex (TDM) traffic to and from the ATM format. Traditional segmenting and reassembling functionality is processor intensive as it must account for channel associated signaling (CAS) values and other superframe information transmitted with TDM traffic. Implementing the SAR functionality on the line cards is expensive as the appropriate circuits must be included on each line card. Moving the SAR functionality to a central switch processor can interfere with normal traffic flows and processor upgrades to handle such additional processing can be prohibitively expensive for many low-cost applications in which access devices are utilized.
In addition to SAR functionality traffic, line cards in an integrated access device must also support inverse multiplexing of ATM (IMA). IMA provides an aggregate bandwidth capacity greater than a single T1 link, but less than a DS-3 link, by carrying a single ATM stream over multiple T1 links. To prevent cells in the stream from being misordered as they are transported over different links having different cell rates, latencies, and/or jitter, the IMA standard uses existing T1 UNI standards for IMA support. In particular, overhead cells are regularly transmitted that indicate the overall order pattern of the cells transported over the different links. This provides an intricate payload block mapping scheme that further increases processor requirements and costs of the line cards.